1. Field of the Invention
The invention relates to a source encoder for video pictures having an encoding unit, and including a control unit which supervises the filling level of a buffer store connected in cascade to an encoding unit and which in dependence upon that filling level, controls a quantizer included in the encoding unit.
2. Description of Related Art
A source encoder with these features is disclosed in an article by G. Lutz et al. (Lutz, G., Speidel, J. und Streicher, E.: Der Videokonferenz-Codec 2Mbit/s-Ein System zur Uebertragung von Bewegtbildern und Grafiken, Sprache und Daten mit 2Mbit/s. PKI Technische Mitteilungen (1985), Vol. 1, pages 50-60). FIG. 1 shows a basic circuit diagram of the prior art source encoder. In this FIG. D denotes a data source producing data from video pictures for an encoding unit C, in which a code conversion of the video data is effected in a bit-saving line code. The encoding unit C is preceded by a pre-processing unit PP. The pre-processing unit PP arranges the video data in blocks: i.e. the data of picture elements which are located in square picture sections. The encoding unit C includes inter alia a picture store in which every T seconds all the blocks associated with a video picture (video pictures must here be understood to mean fields or frames) are stored via a switch S and are intermediately stored until the moment at which they are processed. In addition it also includes a controllable quantizer by means of which the number of bits is controlled by coarse or fine quantization, by which the information components of the blocks are encoded with a more or less great loss of information. This encoded information is written at a bit rate which fluctuates in time in a buffer store PS and, read therefrom at a constant bit rate and transferred to a receiver.
The filling level F of the buffer store PS is continuously supervised by a control unit K. The encoding unit C, the quantizer and the switch S are controlled such, in dependence on the filling level F, that the buffer store PS can neither overflow nor be emptied completely. If, for example, the filling level F of the buffer store PS increases, then the control unit K provides that the quantization stepsize is continuously increased so that consequently the quantization gets coarser. If the filling level decreases the quantization is refined. If in spite of the coarsest quantization the filling level F increases to above the maximum value F.sub.max, the switch S is opened for at least an integral multiple of T seconds, i.e. at least one video picture is omitted from the video picture sequence at the transmitter end (frame or field sub-sampling). This fact is transmitted to the receiver by means of a appropriate information, the video picture received last being repeated on the display screen of this receiver, instead of the missing video picture.
On reaching the filling level F.sub.max the overall encoding circuit C is inoperative for T seconds; then the filling level F of the buffer store PS decreases as it is emptied at a constant bit rate until a lower level F.sub.o. Then the encoding unit C resumes operation, and more specifically--as the filling level increases again--with ever increasing quantization stepsizes, so increasingly coarser quantization, until the filling level F.sub.max is reached again or all the blocks stored in the picture store of the encoding unit C have been processed. When the filling level F.sub.max has been reached again, the encoding unit is stopped again for T seconds. When the stored blocks have been processed, the switch S is closed as soon as the subsequent integral multiple of T seconds after its opening has ended.
The quality of the video picture displayed on the display screen at the receiver end in the prior art encoder needs improvement on the following grounds: During the encoding of a video picture stored in the picture store of the encoding unit C, all the values for the quantization interval are in certain circumstances even cycled-through many times. Consequently, fluctuations between the highest and the poorest quality of individual picture areas occur within a video picture. At the receiver end these circumstances result in that the displayed picture gives an unacceptable total impression for the observer for a short time.